Completed Research
Mass Transfer and Modeling of Liquid-Phase Cyclohexane Oxidation Process in Agitated Reactors
Zeru Berhane Tekie, PhD, 1997
(Thesis: UMI Dissertation Publishing)
The equilibrium solubility (C*), volumetric liquid-side mass transfer coefficient (kLa), Sauter mean diameter (dS), interfacial area (a), gas holdup (εG) and liquid-side mass transfer coefficient (kL) of N2 and O2 in cyclohexane were measured under industrial conditions in 4 liter ZipperClave gas inducing (GIR) and surface aeration (SAR) reactors. The data were obtained in wide ranges of mixing speed (400-1200 rpm), liquid height (0.17-0.27 m) temperature (330-430 K) and pressure (7-35 bar) in both GIR and SAR.
The solubility and kLa values for N2 and O2 in cyclohexane were calculated using a modified Peng-Robinson equation state and the transient physical gas absorption technique. A high shutter speed video camera as well as image processing and analysis system were used to measure gas bubble size distribution using a photographic technique.
The central composite statistical design and analysis technique was used to study the effects of mixing speed, liquid height, temperature and pressure on the mass transfer parameters of N2 and O2. The solubility of both gases were found to increase with pressure as well as temperature and under the same operating conditions O2 was found to have higher solubility than N2 in cyclohexane. Statistical models that correlate kLa at 95% confidence level with the process variables were established for both gases in the GIR and the SAR. Mixing speed and liquid height were found to be the most important process variables affecting kLa, a, εG, and kL of N2 and O2 in both reactors. The bubbles size were found to follow log-normal distribution with a Sauter mean diameter (dS) of 760 µm.
A comprehensive mathematical model using a series of CSTR(s) for the non-catalytic and catalytic liquid-phase cyclohexane oxidation processes was developed using literature kinetic data and the mass transfer parameters obtained in this study under industrial conditions. The effects of mass transfer and process variables on the conversion, yield and selectivity of both processes were demonstrated using the model. An optimization approach based on kLa in each reactor that maximizes the cyclohexanol and cyclohexanone yield and minimizes the by-product yield was also developed for both processes.